September 7, 2011 at 5:46 pm
filed under Rust
Tagged David, Glass-reinforced plastic, Resin, Sandpaper, Staple (wool)
If you do not have a sanding disc for your drill, it is worth buying one, since they are not expensive – and the alternative is to use coarse emery cloth and a sanding block, which is a laborious and time-consuming method.
If the area to be worked on is close to another panel which is not affected by rust, it is as well to use adhesive masking tape to protect it, as a disc sandcr is effective but can be a bit clumsy to use.
You will now be able to see the extent of the damage. If the hole is a tiny one, or if there is a series of holes not much larger than pin holes, it is possible to effect the repair using only body filler.
If the hole or series of holes is larger and the whole area is weakened, one way of bridging the holes and strengthening the surrounding metal is to apply a patch of glass mat. Buy one of the three-part proprietary body repair kits – the type with an inert filler powder, pre-accelerated resin and a paste-type catalyst. A good one is David’s Isopon. This type of kit usually includes some glass mat, probably in two forms – a chopped strand mat and a staple fibre mat. The former is a very coarse mat consisting of random chopped short fibre strands and is normally used for larger repairs. The latter is also a random fibre material but very much finer and of a semi-transparent, cobweb-like appearance. The staple fibre mat is best for tackling fairly small holes or a single small hole.
A substitute for staple fibre mat is a compound such as David’s P.40, whose glass fibre strands are already mixed in the repair paste and only the hardener has to be added.
Preparing the surface
Use a small hammer or a hammer and a small cold chisel to knock out any wafer-thin metal or loose flaking areas from the centre of the rusty area. Then carry on tapping gently until the area around the hole or holes is slightly recessed compared with the surrounding contours. This is to take the repair material. It is important, if additional strength is needed and more than one layer of mat is to be used, that this initial hammering of the area is sufficient to accommodate the extra materials.
Adhesion of glass fibre patches can be assisted further by scouring the metal surrounding the damage with a piece of coarse grade sandpaper. An even better method is to drill a series of small holes to enable the resin to flow into them to provide a really firm fixing.
Before mixing the hardener and resin, prepare the glass fibre patch. Use scissors to cut a piece of the staple fibre mat to shape about 25 mm larger all round than the area to be treated.
Mixing the hardener and resin
follow the instructions for mixing supplied with the kit, but blend only resin and hardener. Do not add the filler powder. Use an old bowl or saucer for the mixing, in which a typical operation would entail tipping a measured quantity of resin into it and adding hardener paste from a tube. Proportions are 25 mm of hardener squeezed from the tube to a tablespoon-ful of resin.*
Setting action starts quickly, particularly in hot weather, so do not mix any more material than you need or can conveniently handle. When mixed, use a cheap paint brush to apply the mix to the entire area of the hole where the glass fibre patch is to go, stippling it well into the pits and dents and around the edge of the hole itself.
Next, place the pre-cut piece of glass mat into position, using the brush to jab it and stipple it well on to the surface. Immediately apply more resin and hardener mix on top, brushing, jabbing and stippling until it is well soaked in the resin and the mat takes on a transparent look.
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If additional strength is required, the whole process can be repeated and a second or even a third layer of glass mat applied. Make sure, though, that each mat is thoroughly soaked in resin and hardener.
Allow the resin and mat to set hard. This should take about 20 minutes but it might take longer if the weather is cold.
The final stage is to use a filler such as David’s P.38 to smooth the surface ready for painting.
Patching on the underside
A slightly different technique can be used, still employing the glass mat, when the rear of the panel is accessible.
The work starts in the same way by scuffing the area of damage clean with a disc sander. Then repeat this operation on the underside of the metal. Exactly the same procedure is then followed to apply one or more glass fibre patches, but this time on the underside of the panel. Adhesion can be helped in the same way as before by scouring the edges of the area before applying the patch. Finish off the repair by applying filler from the front side and sanding smooth.
A good way to support the mat firmly and accurately during the fixing of the patches to the underside is to use a cardboard ‘former’ on the front side of the repair. When the metal is sanded and clean on both sides, cut a piece of card to suit the size and shape of the repair. Cover this with a sheet of polythene or alternatively coat one side of it with wax polish. Then fix the card on the front side of the repair, holding it in position with cellulose tape or masking tape. Ensure it goes on polythene or wax side downwards. When all the glass mat has been laid up on the underside and has set hard, the cardboard can be lifted off and all traces of wax and so on removed from the glass fibre before applying the final filler to the front of the repair.
Using zinc mesh
A popular alternative material used to support the filler when repairing holes is a sheet of perforated metal or mesh.
Where access to the rear of the damaged area is available, the technique is once again to clean back to bare metal to a few centimetres all round the damage on both sides of the panel. Again, it would help also if holes were drilled through the metal around the hole to help key the filler. A piece of mesh or perforated zinc is cut and shaped slightly larger than the hole. If the material is that supplied with the kit, use an old pair of scissors to cut the material – it is quite soft. The next step is to mix up some filler.
When fixing a repair using mesh instead of a glass fibre mat, it is best to use a simple two-part paste filler. There are a number of these on the market. A typical proprietary brand is supplied in two tubes , or in a tin with a tube of hardener. The larger of the Holts tubes and the tin in the David’s kit contain the bulk paste. The mixing instructions are basically the same for both types. The paste and the hardener are mixed together on a non-absorbent surface using a plastic spatula and then applied straight to the work.
Spread a layer of filler all around the mesh and then immediately press the mesh into position. Add more filler to the rear side of the mesh to ensure it is firmly fixed in place. The final filling job can be done from the front.
‘No access’ holes
Where a hole is being filled in some part of the car where access to the other side of the panel is impossible, a slightly different technique will be needed. Cut the piece of mesh in the same way as before and then attach a couple of pieces of wire to it. Then, to make sure the mesh is the correct size, try to ‘fiddle’ it through the hole. This, depending on the shape of the hole, may be possible to do with the mesh in one piece or, alternatively, the mesh may have to be cut into two and each piece inserted separately. When you are satisfied, pull the mesh back through the hole again.
You can now either spread the filler around the mesh and then ‘fiddle’ the mesh back through the hole or, alternatively, spread the filler paste around the edge of the hole at the rear and then push the mesh through the hole. Then pull the wires hard into the layer of paste and hold them tightly until the paste sets firm. Filling is then completed in the usual way from the front.
Methods of supporting repair material
Crude but sometimes effective methods of supporting glass fibre are possible in order to bridge a larger hole in a sill or a section of bodywork where the underside is inaccessible. In some cases, after the area of the hole has been cleaned up and slightly recessed, it is possible to push wire netting or crumpled-up newspapers into the cavity until the level is right to support a glass mat soaked in resin. This will adhere to the front side of the metal and, once again, a few drilled holes and some deep scouring of the metal will aid adhesion.
Using foam support
A modern alternative now available is to fill the cavity or part of the cavity with foam and to use this to support the repair materials. Isovoss is one such foam and all you need to do is simply mix the two liquids together and shake them. The result is instant polyurethane foam which expands very rapidly and then sets hard.
Clean up the area of the rust hole to be filled as before, slightly indenting the surround metal. If the inaccessible cavity underneath the rust hole is a very large one, it is a good idea, if possible, to restrict it so as not to use vast quantities of the foam. In a sill, for instance, crumpled newspaper can be pushed inside and positioned each side of the hole.
When using the foam, the next step will depend a little on which type is used. In general, however, it is a matter of mixing the two liquids in recommended proportions and then shaking or stirring them. Directly the mixture shows signs of taking on a creamy consistency, it must be tipped or injected into the cavity to be filled. It will immediately expand into a large golden mass, filling the cavity and overflowing. Trim off the bulk of the overflow as soon as it begins to set and later, when it is hard, trim off the excess with a sharp knife until the foam is level with the metal. Finally, complete the repair with filler paste.
Aluminium repair tape
One of the simplest rust hole repair methods is to use an aluminium body repair tape. Like the other procedures, start off by cleaning up the metal around the rust damage, getting right back to bright metal and removing all signs of loose and flaking rust. Then use a hammer to recess the area to slightly below the surrounding metal.
The tape is supplied in a roll, usually 50 mm wide with a backing cover. First, offer it up to the repair area and then cut it into the required lengths. Peel off the backing tape and lay each strip into position over the rust holes so that it just meets the surrounding body metal. The tape sets quickly after the backing is removed. So make sure you know exactly where it is going, so it can be applied quickly. Once set, it blends in with the metal and becomes very hard. The final stage is the application of filler.
Impregnated glass fibre cloth
Another self-setting repair material is glass fibre cloth, such as Bondalight, which is impregnated with resin that hardens when exposed to sunlight or an ultraviolet light source. The patch material is pliable while protected by its backing and is self-adhesive.
To use it, clean the damaged area back to bright metal by sanding. Then use a pair of scissors or a knife to cut the Bondalight to size and shape. Next, pull off the green backing, then stick it, tacky side down, over the damaged area. What you need now is sunshine, although Bondalight can be applied to a damp surface. On a good bright day with plenty of sunshine, it will take a couple of hours to harden. If the weather is overcast and there is a temperature of 10°C, hardening could take up to 24 hours. In really cold weather, below 5°C, it is not recommended that Bondalight should be used outdoors. Use your garage, if you have one, and train a high intensity mercury vapour lamp or an ordinary sunlamp on the repair and it will harden in 10 to 15 minutes.
When it has hardened, the clear film which covers the patch material can be removed. Filling, sanding and painting can then follow in the normal way.
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